1. Field of the Invention
This invention relates to an ion implanting system, and more particularly to an ion implanting system equiped with a charge neutralizing facility.
2. Description of the Related Art
Ion implantation is a technique of doping a certain species of ions into a workpiece, used, e.g., for forming regions of desired conductivity of desired type in a semiconductor body. Especially in semiconductor industries, ion implantation is widely used for accurately forming shallow regions of desired impurity concentration.
Ions are charged particles. Ion-implanted workpieces receive the charge of the implanted ions. Further, ions being implanted are energetic particles usually having an energy of several KeV to several hundreds KeV, capable of emitting secondary electrons from the ion-implanted bodies. Thus, there are various charges in an ion implanting system. There arises the problem of charge-up in insulators and isolated conductors disposed in the ion implanting apparatus.
Semiconductor wafers for manufacturing semiconductor devices have insulating bodies such as oxides and isolated conductors such as insulated gate electrodes. Metal-insulator-semiconductor (MIS) structure forms a capacitance C which generates a voltage V upon storing of charge Q, V=Q/C. For example, in implanting positive cations into wafers in an ion implanting apparatus, if a great amount of cations are accumulated on a conducting film disposed on an insulating film, such as a gate oxide formed on the wafer, there is generated a high voltage across a MIS capacitor, which may cause break-down of the nsulating film. Such destruction of insulating films destroys the pattern in the LSI and lowers the yield of LSI manufacture. For protecting workpieces from the damages by the charge-up, it can be considered to reduce the charges in the ion implanting apparatus, or to give charges of opposite polarity in response to charge-ups, thereby neutralizing the charge. Ions to be implanted are in most cases cations which are created by removing electrons from the atoms. Electrons may be superposed on the cations for neutralizing the charge of cations. It has been proposed to provide an electron shower in the vicinity of wafers, to deliver electrons to the wafers being ion-implanted, thereby neutralizing charge of wafers due to the cations. In employing this method, it is desirable to continuously monitor the charge-up of the wafers being ion-implanted, and to control the neutralization to minimize the charge on the wafers. Excessive supply of electrons will charge the wafers negatively.
It has been proposed to dispose a conductor in front of the wafer to oppositely face it and to form capacitive coupling therewith, for monitoring the charge on the wafer. In the case where a plurality of wafers are disposed on a rotating disk and successively subjected to ion implantation, a stationary charge sensor may face and sense the charge on a rotating isolated conductor which approaches and departs the sensor at a relatively high speed. The detection accuracy, however, is not sufficiently high and improvements are highly desired.